Patent GB-1475402-A (GB)
Title: Vector generators
IPC: G09G1/12
Filed: 1974-11-18
Abstract: 1475402 Cathode-ray tube displays SPERRY RAND CORP 18 Nov 1974 [17 Dec 1973] 49843/74 Heading H4T In a vector generator for a cathode-ray tube display (e.g. comprising alpha-numeric characters or graphical symbols) the X and Y deflections, Fig. 2, are produced by linearly charging or discharging respective capacitors at respective rates determined by the inclination # of the vector to the horizontal (i.e. the X) axis. This results in a constant "writing" speed for all vector angles and thus all vectors are of uniform intensity. As shown in Fig. 1, the voltage on respective capacitors 12, 14 determine the X 1  and Y 1  co-ordinates of the instant beam position, i.e. the start point of a vector, which may be the end point of a previous vector. The X 2  and Y 2  co-ordinates of the end point of a new vector to be traced together with X and Y rate signals, cos# and sin#, are then specified in digital form by a computer 46 in response to an END signal (of the previous vector) from a logic control 47 and loaded into D-A converters 36 and 40, in respect of the new X 2  and Y 2  positions and into D-A converters 42 and 44, in respect of the X and Y "writing" rates. The computer then supplies a start signal and clock signals to control logic 47 which now produces a strobe output which causes the D-A converters to produce analogue voltage outputs at points A and C corresponding to the digital inputs. Point A and one plate of capacitor 12 are connected to respective opposite diagonal points of a diode bridge 16 and point C and one plate of capacitor 14 are connected to respective opposite diagonal points of a diode bridge 17. The other diagonal points of bridge 16 are connected to respective controlled current sources 30, 31 and the other diagonal points of bridge 17 are connected to respective controlled current sources 32, 33, sources 30 and 31 being controlled by the analoque voltage output cos# of D-A converter 42 and sources 32 and 31 by the analogue voltage output sin# of D-A converter 44. In operation, if the X and/or Y co-ordinates of the end point of the (new) vector to be traced are more positive than the corresponding coordinates of the start point (which may be the end point of a previous vector) the voltages at A and/or C will be greater than the voltage across capacitor 12 and/or capacitor 14 whereas if either or both of the new end point co-ordinates are less positive than the corresponding start point co-ordinates, the voltages at A and/or C will be less than the voltage across capacitor 12 and/or 14. In the former case, and considering the X co-ordinate only, diodes 21 and 22 are reverse biased and current IX 1 flows from source 30 via diode 20 to charge capacitor 12 linearly at a rate determined by the analogue output of D-A converter 42 until the voltage across the capacitor 12 becomes equal to that at point A. During this charging the X component deflection of the CRT 11, in this case, in the positive direction, is produced. In the latter case, i.e. where the voltage at point A and/or C is less than the voltage across capacitor 12 diodes 20 and 23 are reverse biased and current IX2, at a rate determined by the analogue voltage output of D-A converter 42 flows out of and linearly discharges capacitor 12 via diode 21 and source 31, the X component deflection, in this case, in the negative direction, being produced during such discharge. In either case, when the voltage across capacitor 12 becomes equal to the voltages at point A, the quiescent condition, the current flow divides equally through the two legs of the bridge and maintains the new voltage across the capacitor. In this condition the equality of voltage at points A and B is detected by end detector means 34 which signals this condition to the control logic 47. The Y co-ordinate deflection is produced in similar manner. The controlled current sources may take the form shown in Fig. 3 (not shown).